Water vapor observations up to the lower stratosphere through the Raman lidar during the Maïdo LIdar Calibration Campaign

A new lidar system devoted to tropospheric and lower stratospheric water vapor measurements has been installed at the Maïdo altitude station facility of La Reunion Island, in the southern subtropics. The main objectives of the MAïdo LIdar Calibration Campaign (MALICCA), performed in April 2013, were to validate the system, to set up a calibration methodology, to compare the acquired water profiles with radiosonde measurements and to evaluate its performances and capabilities with a particular focus on the UTLS measurements. Varying the characteristics of the transmitter and the receiver components, different system configuration scenarios were tested and possible parasite signals (fluorescent contamination, rejection) were investigated. A hybrid calibration methodology has been set up and validated to insure optimal lidar calibration stability with time. In particular, the receiver transmittance is monitored through the calibration lamp method that, at the moment, can detect transmittance variations greater than 10-15%. Calibration coefficients are then calculated through the hourly values of IWV provided by the co-located GPS. The comparison between the constants derived by GPS and Vaisala RS92 radiosondes launched at Maïdo during MALICCA, points out an acceptable agreement in terms of accuracy of the mean calibration value (with a difference of approximately 2-3%), but a significant difference in terms of variability (14 vs. 7-9%, for GPS and RS92 calibration procedures, respectively). We obtained a relatively good agreement between the lidar measurements and 15 co-located and simultaneous RS92 radiosondes. A relative difference below 10% is measured in low and middle troposphere (2-10 km). The upper troposphere (up to 15 km) is characterized by a larger spread (approximately 20%), because of the increasing distance between the two sensors. To measure water vapor in the UTLS region, nighttime and monthly water vapor profiles are presented and compared. The good agreement between the lidar monthly profile and the mean WVMR profile measured by satellite MLS has been used as a quality control procedure of the lidar product, attesting the absence of significant wet biases and validating the calibration procedure. Thanks to its performance and location, the MAIDO H2O lidar is devoted to become a reference instrument in the southern subtropics, allowing to insure the long-term survey of the vertical distribution of water vapor, and to document scientific themes such as stratosphere-troposphere exchange, tropospheric dynamics in the subtropics, links between cirrus clouds and water vapor